This invention relates to an ultrasonic receiving apparatus and more particularly to, for instance, a receiving part of an ultrasonic imaging apparatus. The present invention is useful as a receiving beam former or receiving beam deflection circuit for an ultrasonic imaging apparatus.
FIG. 1 shows an arrangement of a conventional receiving beam former comprising transducer elements arranged in an array 1, 2 . . . , N, analog delay circuits 10-1.about.10-N composed of inductance and capacitance, an adder 11 and an output terminal 12. Numeral 16 shows the phase front of receiving signals. Of the elements arranged in an array, since those on the central side receive the signal in an advanced phase as compared with those on the outer side, the receiving signals of all the elements can be added together in an equiphase relationship by delaying those on the central side.
The drawbacks of the conventional receiving beam former include limitations of frequency characteristics, difficulty of micro delay control, greatly enlarged hardware, etc.
On the other hand, as shown in FIG. 2, an arrangement that has once been proposed comprises signal holding means 13-1.about.13-N using capacitors known as sample and hold circuits and a controller 14, whereas control signals and examples of the control signals are represented by C1k.about.CNk and 17-1.about.17-N, respectively.
All the receiving signals can thus be coherently added by sampling the receiving signals on a concave line corresponding to the time at which the receiving signals reach the transducer elements and adding the sampled signals together.
Referring to FIGS. 3A and 3B, problems inherent in the conventional method will be described. Cik represents the control signal of the sample and hold circuit 13-i of FIG. 2, where i=1.about.N (N is the number of transducer elements) and k=1.about.M (M is a number indicating a time series). T represents a sampling period indicating, for instance, the value of 100.about.500 ns. .tau..sub.o represents the settling time of the sample and hold circuit, its value being roughly equal to 10 ns, for instance. Ci'k' represents a control signal of the receiving signal of the transducer element different from Cik and the time difference between Cik and Ci'k' is assumed to be .tau.. .tau..sub.o, .tau. are shown in enlarged scale compared with T for convenience of illustration.
FIG. 3A indicates a case where a receiving signal .alpha. is ideally sampled and held, whereby an output .beta. is obtained. In this case, the sample and hold output is in a transitional state within the range from the control signal Cik to the settling time .tau..sub.o and the signal becomes correct after the time .tau..sub.o.
However, in case the control signal Ci'k' of the receiving signal of the transducer element of i' different from i exists in Cik the time .tau. (.tau.&lt;.tau..sub.o) later, noise will be mixed with the receiving signal .alpha. because of electric coupling and the mixture will become a receiving signal .alpha.'.
Since the time at which the noise appears is less than the settling time of the sample and hold circuit, the output of the sample and hold circuit becomes .beta.' as shown in FIG. 3B, causing an error .epsilon. between .beta.' and the ideal output .beta.. As a result, problems are posed on the operation of the receiving beam former relative to the dynamic range of the signal, S/N ratio and the like.